1. Field of the Invention
In general, this invention relates to compositions and articles suitable for use in thermal transfer imaging processes, and also relates to graphic articles comprising a graphic image formed using the inventive compositions and articles, and methods of making such graphic articles.
2. Related Art
Multilayer graphic articles may be applied to a variety of surfaces for decorative, informational, and/or functional reasons. These multilayer constructions often contain one or more continuous or non-continuous color layers which have been thermally transferred from a carrier film, or coated, printed, or laminated onto a substrate.
Decorative graphic articles are typically highly contrasting polychromatic constructions that enhance the visual appeal of surfaces to which they are applied, such as motor vehicles, marine craft, commercial or residential real estate, signs, store displays and the like. Informational graphic articles provide directions, location indicia, instructions, and identification when used, for example, to construct road signs and license plates. Functional graphic articles impart weather protection and wear resistance to surfaces to which they are applied, especially outdoor surfaces.
Color layers in presently known graphic articles often contain a poly(vinylchloride) ("PVC") binder blended with various color agents, volatile organic solvents, and plasticizers. PVC binders, which typically make up 30 to 50 weight percent of a color layer formulation, are not considered to be environmentally friendly. Volatile organic solvents typically provide 40 to 60 weight percent of a formulation. For various environmental and health reasons, the reduction or elimination of these solvents is desirable. Reduction or elimination of the use of PVC plasticizers is also desirable. Plasticizers can migrate in adjacent layers of the graphic article and cause visual changes to both the color layer and surrounding members which could adversely affect the stability of the color layer or appearance of the graphic article.
Presently known color layer formulations are compatible with only a limited class of substrates, primarily PVC, acrylics, and urethanes. These substrates can have poor flexibility and PVC is not environmentally desirable.
Accordingly, a substantial need exists for eliminating or reducing the use of PVC-based materials (and their associated solvents and plasticizers) in both color layer formulations and graphic articles, and in articles and methods used for their production, e.g., donor elements such as thermal transfer ribbons and hot stamp foils.
It has long been known to provide retroreflective articles with cover films (for example, made of polymethylmethacrylate, polyvinylchloride, polyester, etc.) to improve the retroreflective performance which is provided under wet conditions and to protect the retroreflective elements. See, for example, U.S. Pat. No. 2,407,680 (Palmquist et al.), U.S. Pat. No. 3,190,178 (McKenzie), and U.S. Pat. No. 4,025,159 (McGrath). To achieve improved durability, embossability, and abrasion resistance, improved retroreflective sheetings with new selections of materials for cover films are now known, e.g., including ethylene/acrylic acid copolymers as disclosed in U.S. Pat. No. 4,664,966 (Bailey et al.), U.S. Pat. No. 4,767,659 (Bailey et al.), U.S. Pat. No. 4,896,943 (Tolliver et al.), and U.S. Pat. No. 5,066,098 (Kult et al.). A common problem with such cover film materials is that in order to achieve satisfactory adhesion of imaging materials thereto, such as is used to print legends on and/or color the retroreflective article, surface priming layers and/or techniques must be employed. The need exists for ink formulations and thermal transfer articles, e.g., thermal transfer ribbons and hot stamp foils, having color layers that can be readily and easily applied to such cover films without using priming layers or techniques.
It is well known that graphic articles can be prepared by imagewise transfer of color layers by application of heat and/or pressure. Two examples of this are the "hot stamp" application, and thermal mass transfer. A limitation of typical thermal transfer color layers is the poor durability resulting from the low cohesive strength necessary to perform distinct imagewise transfer.
A need exists for improving the durability of images formed by thermal transfer methods.
One known method described in U.S. Pat. No. 5,037,447 (Nishimoto et al.) involves using a chemical agent to treat the image after it has been transferred to the receptor substrate. While useful in its ability to improve the cohesive strength and resulting durability of the material, this method is limited by the requirement of using a potentially irritating and messy chemical agent.
It is known to form graphic patterns on substrates using transfer articles bearing predesignated designs. G.B. Patent No. 1,218,058 (Hurst et al.) discloses transfers with an adhesive layer applied to only those areas intended to be transferred to the substrate; U.S. Pat. No. 4,786,537 (Sasaki) and U.S. Pat. No. 4,919,994 (Incremona et al.) disclose transfer graphic articles wherein the graphic design is formed via imagewise differential properties within the transfer film itself. One problem with such approaches is that a large and varied inventory must be maintained in order to provide a variety of graphic patterns.
Hot stamping foils comprising a carrier, one or more color layers, and an adherence layer have been known for some time. Such films have been used to provide imagewise graphic patterns, e.g., alphanumeric or decorative legends, to substrates via imagewise application of heat and/or contact or pressure. In some embodiments, additional members such as release layers are used to facilitate desired performance. In some embodiments, so-called "texture layers" and/or "ticks", metal layers, etc. are used as well to yield desired appearance. Hot stamping foils are also sometimes called hot stamp tapes or thermal transfer tapes. Other related thermal mass transfer methods are known, e.g., using thermal transfer ribbons comprising a carrier releasably bonded to a color layer that is thermally transferred to a desired substrate.
The color layer(s), adherence layer (if any), and any other layers (if any) of the thermal transfer element that are to be selectively applied to the substrate should split or fracture in desired manner in order for the applied graphic pattern to have a desired edge appearance. Some illustrative examples of previously known hot stamping foils are disclosed in U.S. Pat. No. 3,770,479 (Dunning) U.S. Pat. No. 3,953,635 (Dunning), and U.S. Pat. No. 4,084,032 (Pasersky).
An advantage of the foregoing techniques is that the transfer film may be made as a uniform sheet, i.e., with no specific latent image embodied therein. The applicator defines the graphic pattern by controlling the application process, e.g., imagewise application of heat and/or contact pressure. This permits maintenance of a smaller inventory of thermal transfer element material.
One well known use of hot stamping foils is to print legends on vehicle identification plates. For example, license plates produced using hot stamping foils have been used in Austria, Australia, Finland, Germany, Ireland, Portugal, and Switzerland. One commercially available hot stamping foil currently used on license plates with polyvinyl chloride cover films is believed to comprise a polyester carrier, about 28 microns thick; a color layer based on acrylic resins such as polymethyl methacrylate and containing carbon black pigments, about 5 microns thick; and an acrylate-based adherence layer, about 5 microns thick. Examples of resins that are believed to have been used in adherence layers include polyvinyl alcohol copolymers, nitrocellulose, and methyl methacrylate/butyl methacrylate copolymers.
Recently improved retroreflective sheetings have been made available which have cover films made of olefin-based materials or polyurethane-based materials to improve certain performance. As disclosed in the aforementioned U.S. Pat. No. 4,896,943 (Tolliver et al.), olefin-based cover films, e.g., ethylene/acrylic acid copolymers, can provide superior properties including abrasion and dirt resistance. Many conventional hot stamping foils do not achieve good adherence to such cover sheets, however, resulting in graphic patterns having unsatisfactory durability and performance.
More recently, U.S. Pat. No. 5,393,950 (Caspari) discloses hot stamping foils well suited for use on retroreflective articles wherein the foils comprise a carrier, optionally a release control layer, a color layer, and an adherence layer wherein the adherence layer comprises, and may consist essentially of, a mixture of an ethylene copolymer dispersion and an acrylic dispersion.
The need exists for improved thermal transfer articles which can be used to form durable graphic patterns on such cover sheets and a method for forming such graphic patterns.